Rotary screen printer auxiliary conveyor drive to eliminate creep

ABSTRACT

A rotary screen printing machine comprising at least one thinwalled cylindrical stencil operative in a printing path defined by the upper flight of an endless belt guided along two turning wheels, at least one of which is positively driven, a pair of stiff rollers being in pressing contact with the belt in a position ahead of the stencils, one or both rollers and all the stencils being operatively connected to said driven turning wheel.

United States Patent Vertegaal Sept. 9, 1975 [54] ROTARY SCREEN PRINTERAUXILIARY 2,574,002 I 11/1951 Weiss 101/247 X CONVEYOR DRIVE o ELIMINATECREEP 2,735,537 2/l956 Keller I 198/203 2,809,582 lO/1957 Van der W1ndenlO1/248 X [75] Inv ntor: Ja us rardus gaal, 2,973,084 2/1961 Sinden eta1. 198/208 x Boxmeer, Netherlands 3,420,167 1/ 1969 Vander Winden101/116 3,426,680 2/1969 Kaufmann 101/181 [731 Asslgnee: Amsterdam3,807,549 4 1974 Cowdery et a1 198/203 Amsterlveen, Netherlands 22Filed: Oct. 31 1973 1 Primary ExaminerEdgar S. Burr PP 411,338 AssistantExaminerR. E. Suter Related Application Data Attorney, Agent, orFirm-1rving Weiner [63] Continuation-in-part of Ser. No. 125,085, March17,

1971, abandoned.

[57] ABSTRACT [30] Foreign Application Priority Data Aug. 25, 1972Netherlands 7204068 A rotary screen printing machine comprising at leastone thin-walled cylindrical stencil operative in a print- [52] US. Cl101/118; 101/126 i th defined by the upper flight of an endless belt [5Int. CI.2 along two turning wheels at least one of 1 Field of Search101/1 l5, 1 1 126, is positively driven, a pair of stiff rollers beingin press- 198/203, 208 ing contact with the belt in a position ahead ofthe stencils, one or both rollers and all the stencils being ReferencesCited operatively connected to said driven turning wheel.

UNITED STATES PATENTS 2,359,825 10/1944 Campbell 101/115 10 Claims, 1Drawing Figure PATENTEDSEP' ems INVENTOR JACOBUS GERARDUS VERTEGAALTTORNEY ROTARY SCREEN PRINTER AUXILIARY CONVEYOR DRIVE TO ELIMINATECREEP CROSS-REFERENCE TO RELATED APPLICATIONS This application is acontinuation-in-part application of US. Pat. Application Ser. No.125,085, filed Mar. 17, 1971 for ROTARY SCREEN PRINTING MA- CHINE andnow abandoned.

BACKGROUND OF THE INVENTION 1. Field of the Invention This inventionrelates to a rotary screen printing machine comprising at least onethin-walled cylindrical stencil which is operative in a printing pathconstituted by a driven endless belt or supporting blanket disposedaround two turning wheels one of which is situated at the beginning andthe other of which is situated at the end of the printing path, one ofthe wheelsbeing provided with driving means and with a drivingconnection with each stencil. Such a machine is known in manyembodiments.

2. Prior Art In order to obtain an accurate printing, it was suggestedin US. Pat. No. 3,420,167, issued on Jan. 7, 1969 to dimension thedriving connection in such a manner that in the area of contact betweeneach stencil and the belt, the linear velocity of the belt is slightlygreater than the tangential velocity of the stencil. It was an object ofthis feature to avoid the detrimental consequences of the normallyexisting clearance or play in the driving means so as to minimize thechanging of the friction forces between the material to be printed andthe screens due to the force exerted on them by the squeegee, therebyincreasing screen life.

There remains, however, the small fluctuations of velocity of the belt,which occur owing to the fact that the composition of the belt is notfully homogeneous as a result of which the neutral plane or surface ofthe belt, which is that plane where the length of the belt remainsunchanged and, hence, carries no stress, is not always at the samedistance from the surfaces of the belt which is in contact with theturning wheel. This neutral plane indicates the area of the belt whichon passing over the turning wheel is not subjected to a change oflength. The material of the belt which is situated between the neutralplane and the outer surface of the belt away from the turning or drivenwheel is tensioned, while the material inwards of the neutral planetoward the driven wheel is compressed. This causes a displacement of thebelt along the circumference of the turning wheel relative to theturning or driven wheel which displacement or slippage between the beltand the driven wheel may be termed creep. The direction of this creepdepends on the tensile force produced in the upper and lower flight ofthe moving belt.

SUMMARY OF THE INVENTION It is an object of this invention to eliminateor reduce the symptom of relative displacement of the belt and drivenwheel. To that end the machine according to the invention isdistinguished in that as contemplated in the direction of advance of thebelt a rigid pair of rollers is provided at a location ahead of orupstream of the first stencil, at least one of the two rollers having adriving connection with the driven turning wheel and both rollers beingadapted to be vigorously pressed toward each other, while the belt islying between them.

Due to these features an area of line contact is formed between the beltand the rollers to allow the rollers and the belt to have an identicalspeed, the belt thereby attaining a speed which is equal to thetangential velocity of the driven roller. The creep" situation of thebelt due to the varying position of the neutral plane relative to thedriven wheel no longer exerts any detrimental influence. Usually, whenthe neutral plane moves in an outward direction away from the driventurning wheel, which movement is due to manufacturing variations in thebelt, as the belt moves over the circumference of the driven turningwheel, the linear speed of the belt increases. At the same time thetension in the upper flight, or running on part, between the driventurning wheel and the fixed position pair of rollers increases. Thiscauses the creep occurring along the circumference of the driven turningwheel due to the compression in the portion of the belt between theneutral plane and the surface of the belt in contact with the driventurning wheel, to move in a direction contrary to the rotationaldirection of the driven wheel or toward the upper flight of the belt.This creep toward the upper flight of the belt in turn causes a decreasein the tension of the upper flight. On the other hand, when the neutralplane moves inwardly, or closer to the driven turning wheel, which isagain due to manufacturing variances of the belt, the linear speed ofthe belt decreases, the tension in the upper flight decreases andincreases in the lower flight. This causes the creep along thecircumference of the driven turning wheel to move in the direction ofrotation of the turning wheel, or toward the lower flight of the belt.The constant changes of fluctuations in the linear speed of the beltimpairs the quality of printing produced on the printing machine. Thepair of rollers located upstream of the first stencil insure a constantspeed of the belt by functioning as auxiliary driving wheels to thedriven turning wheel by forcing the upper flight portion of the belt tomove at the desired constant speed.

It should be noted, that this effect could theoretically also beobtained by driving the supportng belt or blanket only via the pair ofrollers. However, a considerable amount of power is required to drivethe belt and it is not feasible with the line contact between the pairof rollers and the belt to develop the needed frictional forces to drivethe belt using the rollers alone. The provision according to theinvention, i.e., the combined drive by the turning or driven wheel andthe pair of rigid rollers, eliminates entirely the harmful effect of thecreep and requires supplying only a small amount of power to the pair ofrollers.

For a more complete understanding of the present invention reference ismade to the following detailed description and accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING The drawing diagrammaticallyillustrates the most essential parts of a rotary screen printing machineembodying the principles of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT Now, with reference to thedrawing, there is depicted a rotary screen printing machine. Thedepicted machine is provided with three thin-walled cylindrical stencils1, which are operative in a printing path 2 constituted by a drivenendless belt or supporting blanket 3, generally manufactured from rubberor the like, and which is disposed around two turning wheels 4 and 5.The wheel 4 is situated at the beginning and the wheel 5 at the end ofthe printing path 2. The turning wheel 5 is provided with driving means6 and with a driving connection 7 to each stencil 1. In a manneranalogous to that described in US. Pat. No. 3,420,167, the turning wheel5 is driven by the means 6 via a worm gear 8. Each stencil l is drivenvia a worm gear 9 which in conformity with the above-mentioned patent isdimensioned with respect to the driving worm gear 8 such that aparticular desired ratio of speed is obtained between the stencil andthe belt in the printing area, i.e., the tangential velocity of eachstencil is equal to the tangential velocity of the turning or drivenwheel 5.

The provision according to the present invention consists of a pair ofstiff or rigid rollers 10 and 1 1 which, as contemplated, in thedirection of advance of the belt 3, are disposed ahead or upstream ofthe first stencil at the beginning of the printing path 2 opposite thatend thereof at which the driven turning wheel 5 is located. The upperroller 10 has a driving connection 7 with the driven turning wheel 5,via a worm gear 12. The two rollers 10, 11 can be vigorously pressedtowards one another, while the belt 3 is lying between these rollersthus exerting a force on the belt normal to the longitudinal axis of thebelt to provide an adequate direction force between the rollers 10, 11and the belt so as to cause the belt'to have the same velocity as therollers. The lower roller 1 1 has a diameter smaller than that of theroller 10. Furthermore, the tangential velocity of the roller 10 isequal to the tangential velocity of the wheel 5.

In practicing the present invention the rigid or stiff rollers 10, 11are made from any suitable metal. Optionally, the metal may have a thincoating of any suitable hard plastic deposited thereon. The plasticcoating increases the frictional contact between the rollers and thebelt. The rollers may be either a solid mass or hollow. When the rollersare hollow they are provided with a wall thickness substantially greaterthan that of the stencils. Generally, stencils are thin-walled membershaving a wall thickness of about 0.003 inches. The wall thickness of therollers hereof is greater than the thickness of the walls of thestencils. In this regard it is to be noted that the stencils aregenerally fabricated from nickel, the Squeegees are generally fabricatedfrom a thin strip of spring steel, and the counter-rollers below thebelt 3 are usually manufactured from ordinary metal.

The rollers 10, 11 which are biassed towards the belt exert a force orpressure on the belt normal to the longidudinal axis thereof rangingfrom about 1 to 2 kilograms per centimeter of width of the belt. Thus,for example, a belt having a width of 120 centimeters will have apressure or force of from about 120 to 240 kilograms applied thereto atthe area of contact of the rollers 10, 11. Because the rollers and thebelt are dry at the area of contact, a great frictional resistance isgenerated between the rollers and the belt, i.e., no slip.

The pressure exerted by the rollers 10, l 1 is quite distinct from anypressures exerted by the stencils and their associated internalSqueegees. The stencils, per se, do not exert any pressure on the belt.Rather, it is only the internal squeegee 14 which is pressed downwardly.

However, the force applied to the squeegee when pressed downwardly isonly of the magnitude of about 0.5 kilograms per centimeter of length.The counterrollers apply an equal and opposite reaction force. However,there is little or no frictional force or resistance generated betweenthe squeegee l4 and the inner surface of the rotating stencil associatedtherewith because of the printing paste under the convex face of thesqueegee. The presence and use of the rigid rollers, as noted,eliminates any speed fluctuations or variations in the belt.

The arrangement is such that the stencils l are lying in the portion ofthe printing path 2 situated between the pair of rigid rollers 10, 11and the driven turning wheel 5. The driven wheel 5 has a diameter whichis at least 250 times the thickness a of the belt 3 to minimizefluctuations in the linear speed of the belt. In this embodiment theupper flight of the belt 3 can be considered as the so-called running-onpart, while the lower flight of the belt 3 constitutes the running-offpart. In this lower flight some tension rollers 13 are provided. Asqueegee 14 is provided within each stencil 1.

In operation the turning or driven wheel 5, the stencils l and theroller 10 are driven. The rollers 10, 11 drive the belt in conjuctionwith the driven wheel 5, i.e., they draw the lower flight of the belt 3in the direction of the wheel 5. A certain tensile stress in the upperand in the lower flight of the belt 3 is produced, the tension in theupper flight normally being greater than in the lower flight. As aconsequence of the inevitable uneveness in the composition of the belt3, the neutral plane S, which is an imaginary surface interior to thebelt 3 wherein the belt material remains unchanged in length as the beltis made to curve and hence a surface which carries zero stress willevery now and then be closer or farther from the center 15 of the wheel5. The neutral plane S represents the imaginary plane within the belt 3which has a velocity corresponding exactly with the angular velocity ofthe turning wheel 5. Along the circumference of the wheel 5 the materialof the belt 3 between the surface of the belt in contact with the wheeland the neutral plane S of the belt is compressed and the creepphenomenon occurs. When the neutral plane S shifts outwardly, due to thecomposition of the belt 3, the speed of the belt increases and the upperflight becomes more tensioned. Owing to the difference in tensile stressof the lower and upper flights of the belt 3, the creep will golongitudinally of the belt in the direction of the upper flight. As aconsequence the tensile stress in this upper flight diminishes but inthe lower flight it increases. As a result, the velocity of the beltadjusts itself in a correcting sense and becomes equal to the constanttangential velocity determined by the roller 10.

If a shifting of the neutral S occurs inwardly, the speed of the beltdecreases and the tension of the lower flight will increase. The creepproduced along the circumference of the turning wheel 5 advanceslongitudinally in the direction of the lower flight, an analogouseffect, but in the opposite sense will be produced and finally avelocity of the belt will be obtained and maintained which correspondsexactly with the circumferential velocity of the roller 10. The powerrequired to drive the roller 10 is very small and is a fraction of thepower supplied to the turning wheel 5.

then the diameter of the wheel 4 must be greater than shown in thedrawing but the arrangement of the pair of rollers 10, 11 remains inconformity with the embodiment described.

It is to be understood that by the practice of the present invention,and due to the forces exerted by the rollers l0, 11 on the belt, thetraveling speed of the belt is equal to the tangential speed of therollers. Moreover, the driven turning wheel, which is connected to thedriving means, operates the rollers responsively to the speed at whichthe belt rotates around the driven wheel. This responsive operationcompensates for variations in the distance of the neutral plane of thebelt from the center of the driven wheel, thus, permitting the belt totravel at a constant linear velocity.

Having thus described the invention, what is claimed 1. A rotary screenprinting machine comprising at least one thin-walled cylindrical stencilmounted for rotational movement around its longitudinal axis andoperatively disposed in a printing path, two spaced apart rotatablymounted turning wheels, an endless belt disposed about the two turningwheels having an upper flight portion one surface of which constitutesthe printing path of the printing machine, driving means operativelyconnected to one of the turning wheels, a pair of rigid rollers beingdisposed in the upper flight portion of the belt, one roller of saidpair contacting the printing path, and the other roller being disposedon the opposite surface of the upper flight portion, each of the rollersof the pair of rigid rollers being biassed toward the belt to exert aforce thereon normal to the longitudinal axis of the belt to eliminateslippage between the belt and the pair of rollers, driving meansoperatively connected to at least one roller of the pair of rollers, theturning wheel to which the driving means is operatively connected andthe pair of rollers cooperate with each other to maintain at least inthe upper flight portion of the belt a constant liner velocity.

2. A rotary screen printing machine according to claim 1 wherein saidone roller of the pair of rigid rollers which is disposed in the upperflight portion of the belt has a larger diameter than said other rollerof the pair of rollers which is disposed on the opposite surface of theupper flight portion of the belt.

3. A rotary screen printing machine according to claim 1, wherein thediameter of the turning wheel to which the driving means is operativelyconnected has a larger diameter than the other turning wheel.

4. A rotary screen printing machine according to claim 3, wherein thediameter of the turning wheel to which the driving means is operativelyconnected is at least 250 times the thickness of the belt.

5. The rotary screen printing machine of claim 1 wherein said pair ofrigid rollers exerts a force on the belt of l to 2 kilograms percentimeter of width of the belt.

6. The rotary screen printing machine of claim 1 wherein the rollerscomprise a metal having a hard plastic coating deposited thereon.

7. A rotary screen printing machine comprising, in combination:

a frame;

a drive turning wheel and an idler turning wheel rotatably mounted onsaid frame in horizontally spaced relation;

an endless belt trained about said turning wheels and presenting anupper flight for supporting and carrying along material to be printedupon;

at least one cylindrical stencil rotatably mounted on said frame andextending transversely of said upper flight of said belt and having apredetermined pattern for engaging and printing upon said materialcarried along by said belt;

one and only one pair of rigid pressure rollers contacting said upperflight at a location which is before the location for any of saidengaging and printing upon said material carried along by said belt;

said rigid pressure rollers being firmly and vigorously pressed towardseach other while said endless belt is disposed between said rollers forexerting a force on said belt which is perpendicular to the longitudinalaxis of said belt so that the speed of said belt will always beidentical to the tangential speed of said rigid pressure rollers;

drive means for rotating said drive turning wheel, each said cylindricalstencil, and at least one of said rigid pressure rollers;

one of said rigid pressure rollers having a diameter which is largerthan the diameter of the other one of said rigid pressure rollers; and

said endless belt is moved by the combined drive of said drive turningwheel and the driven rigid pressure roller to eliminate the effect ofcreep between said endless belt and said drive turning wheel.

8. A rotary screen printing machine according to claim 7, wherein saidpair of rigid pressure rollers exerts a force on said endless belt of 1to 2 kilograms per centimeter of width of said endless belt.

9. A rotary screen printing machine according to claim 7, wherein thediameter of said drive turning wheel is at least 250 times the thicknessof said belt and is less than 300 times the thickness of said belt.

10. A rotary screen printing machine according to claim 7, wherein saidpair of rigid pressure rollers acts on said belt in conjunction withsaid drive turning wheel to move and control the speed of said belt andto force said upper flight of said belt to move at a constant speed.

1. A rotary screen printing machine comprising at least one thin-walledcylindrical stencil mounted for rotational movement around itslongitudinal axis and operatively disposed in a printing path, twospaced apart rotatably mounted turning wheels, an endless belt disposedabout the two turning wheels having an upper flight portion one surfaceof which constitutes the printing path of the printing machine, drivingmeans operatively connected to one of the turning wheels, a pair ofrigid rollers being disposed in the upper flight portion of the belt,one roller of said pair contacting the printing path, and the otherroller being disposed on the opposite surface of the upper flightportion, each of the rollers of the pair of rigid rollers being biassedtoward the belt to exert a force thereon normal to the longitudinal axisof the belt to eliminate slippage between the belt and the pair ofrollers, driving means operatively connected to at least one roller ofthe pair of rollers, the turning wheel to which the driving means isoperatively connected and the pair of rollers cooperate with each otherto maintain at least in the upper flight portion of the belt a constantliner velocity.
 2. A rotary screen printing machine according to claim 1wherein said one roller of the pair of rigid rollers which is disposedin the upper flight portion of the belt has a larger diameter than saidother roller of the pair of rollers which is disposed on the oppositesurface of the upper flight portion of the belt.
 3. A rotary screenprinting machine according to claim 1, wherein the diameter of theturning wheel to which the driving means is operatively connected has alarger diameter than the other turning wheel.
 4. A rotary screenprinting machine according to claim 3, wherein the diameter of theturning wheel to which the driving means is operatively connected is atleast 250 times the thickness of the belt.
 5. The rotary screen printingmachine of claim 1 wherein said pair of rigid rollers exerts a force onthe belt of 1 to 2 kilograms per centimeter of width of the belt.
 6. Therotary screen printing machine of claim 1 wherein the rollers comprise ametal having a hard plastic coating deposited thereon.
 7. A rotaryscreen printing machine comprising, in combination: a frame; a driveturning wheel and an idler turning wheel rotatably mounted on said framein horizontally spaced relation; an endless belt trained about saidturning wheels and presenting an upper flight for supporting andcarrying along material to be printed upon; at least one cylindricalstencil rotatably mounted on said frame and extending transversely ofsaid upper flight of said belt and having a predetermined pattern forengaging and printing upon said material carried along by said belt; oneand only one pair of rigid pressure rollers contacting said upper flightat a location which is before the location for any of said engaging andprinting upon said material carried along by said belt; said rigidpressure rollers being firmly and vigorously pressed towards each otherwhile said endless belt is disposed between said rollers for exerting aforce on said belt which is perpendicular to the longitudinal axis ofsaid belt so that the speed of said belt will always be identical to thetangential speed of said rigid pressure rollers; drive means forrotating said drive turning wheel, each said cylindrical stencil, and atleast one of said rigid pressure rollers; one of said rigid pressurerollers having a diameter which is larger than the diameter of the otherone of said rigid pressure rollers; and said endless belt is moved bythe combined drive of said drive turning wheel and the driven rigidpressure roller to eliminate the effect of creep between said endlessbelt and said drive turning wheel.
 8. A rotary screen printing machineaccording to claim 7, wherein said pair of rigid pressure rollers exertsa force on said endless belT of 1 to 2 kilograms per centimeter of widthof said endless belt.
 9. A rotary screen printing machine according toclaim 7, wherein the diameter of said drive turning wheel is at least250 times the thickness of said belt and is less than 300 times thethickness of said belt.
 10. A rotary screen printing machine accordingto claim 7, wherein said pair of rigid pressure rollers acts on saidbelt in conjunction with said drive turning wheel to move and controlthe speed of said belt and to force said upper flight of said belt tomove at a constant speed.